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Deliverability Letter

452 4.5.3 (ATTR38)

452 4.5.3 ATTR38 Mixed Regions

Region Mismatch

The recipient list spans multiple Microsoft regions and cannot be processed in one transaction.

Multi-tenant organisations often host users across regions. Microsoft requires you to send separate transactions per region.

Severity

WARNING

Provider

Microsoft 365 / Outlook

Reference

ATTR38

Last Reviewed

2025-06-01

How to Fix 452 4.5.3 (ATTR38)

Follow these steps to resolve this Microsoft 365 / Outlook SMTP error and restore normal delivery.

  • ✓Segment distribution lists so each message targets one Microsoft region.
  • ✓Review hybrid routing or smart hosts that override EOP’s decisions.
  • ✓Ask the recipient organisation to consolidate MX routing where possible.
  • ✓Leverage per-region connectors in Microsoft 365 if you manage the tenant.

Helpful Resources

More Microsoft 365 / Outlook SMTP Codes

Keep these related troubleshooting guides handy while addressing 452 4.5.3 (ATTR38).

421 4.7.0 – 421 4.7.0 Temporary System Problem
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451 4.4.4 (ATTR5) – 451 4.4.4 ATTR5 Temporary Server Error
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451 4.7.500 (S77719) – 451 4.7.500 Server Busy (S77719)
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451 4.4.62 (ATTR35) – 451 4.4.62 ATTR35 Wrong Region
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